1. Field of the Invention
This invention relates to an intake air amount-estimating apparatus for internal combustion engines, which estimates the amount of intake air drawn into the engine.
2. Prior Art
Recently, it has been proposed to estimate the amount of intake air drawn into an internal combustion engine by the use of equations based on a fluid dynamics model applied to the intake system of the engine. The present assignee has also proposed by Japanese Laid-Open Patent Publication (Kokai) No. 6-74076 a technique of this kind which determines the amount of intake air flowing through a throttle valve, which is considered as a restriction, from a pressure difference between upstream and downstream sides of the throttle valve by the use of a theoretical formula of a throttle-type flowmeter, and calculates an amount of intake air drawn into the cylinder based on the estimated amount of intake air.
However, such a fluid dynamics model is formulated on the assumption that the object (intake system of the engine in the present case) is in an ideal state, and it is difficult to accurately determine various constants, such as a specific heat, which were used in formulating the model, when the object is in an actual state. Further, the equations based on the fluid dynamics contain powers, roots, etc., requiring calculations of them. Therefore, approximate values are applied to the equations in actual calculations. For these reasons, it is impossible to completely eliminate errors in implementing the model on an actual object.
Therefore, it is still critically important to reduce the difference or error between the estimated amount of intake air drawn into the cylinder determined by the model and the actual amount of intake air drawn in the cylinder. To this end, the present assignee has proposed by Japanese Laid-Open Patent Publication (Kokai) No. 7-42600 a method which is based on the fluid dynamics model but can absorb errors in implementing the equations of the model to thereby obtain more accurate estimated values of the amount of intake air drawn into the cylinder without the need of complicated calculations and irrespective of whether the engine is in a transient operating condition, deterioration, aging or variations of component parts and elements of the engine.
All the proposed methods given above estimate the amount of intake air drawn into the cylinder, based on the degree of opening of the throttle valve. On the other hand, there can occur a phenomenon that even when the engine is in a steady operating condition, the throttle valve opening undergoes slight changes due to vibrations of the engine transmitted to a throttle body in which the throttle valve is mounted. If such changes in the throttle valve opening are detected and input to an intake air amount-estimating apparatus, the intake air amount is estimated to values corresponding to such changes. As a result, the actual operation of the engine is not stabilized in spite of the engine being in a steady operating condition, leading to degraded driveability of the engine. Moreover, the throttle valve opening .theta.TH and the intake air amount Gc are in a relationship as shown in FIG. 25, which shows that a slight change in the throttle valve opening while the throttle valve assumes a small degree causes a large change in the intake air amount Gc, so that the intake air amount Gc is estimated to have largely changed. Therefore, there is a demand for stable and accurate estimation of the intake air amount, particularly when the throttle valve assumes a small degree of opening.